1. Field of the Invention
The present invention relates to a receiver for use in a spread spectrum communication system and, particularly, to a receiver for use in a CDMA (code division multiple access) communication system for modulating an information signal with a spreading code which is a signal having a rate several tens to several hundreds times a rate of the information signal and transmitting the modulated information signal.
2. Description of Related Art
One of related prior art is disclosed in, for example, Japanese Patent Application Laid-open No. H8-116303, entitled "Spread Spectrum Radio Communication System and Radio Communication Devices for Use in a Base Station and Mobile Stations of the Communication System", especially in line 10 of page 12 to line 47 of page 13 and in FIG. 10 of that desclosure.
A receiver for use in a CDMA system utilizing a spread spectrum scheme requires a searcher circuit for continuously catching a timing of the spreading code contained in a receiving signal such that a receiving can be performed on the side of the receiver with an appropriate timing. The timing of the spreading code is usually obtained by calculating a mutual correlation between the receiving signal and a replica of the spreasing code on the side of a transmitter. Since the value of mutual correlation becomes maximum when the timing of the spreading code exactly coincident with the receiving timing, the receiver can know the timing of the spreading code of the receiving signal by monitoring the mutual correlation value.
Further, the receiver receives a receiving signal through a RAKE synthesis performed on the basis of an output value of the searcher circuit. In the signal receiving through the RAKE synthesis, the quality of receiving signal is maintained by demodulating signal components passed through a plurality of signal paths having different time delays and received by the receiver at timings corresponding to the time delays and synthesizing the demodulated signal components.
FIG. 1 shows a construction of the receiver disclosed in the Japanese Patent Application Laid-open No. H8-116303, as a typical example of the RAKE receiver. The receiver shown in FIG. 1 is constructed with an antenna 201, a radio frequency demodulator circuit 202, finger circuits 203a to 203n, a finger information extractor 204 and a symbol synthesizer 205.
Now, an operation of this prior art receiver will be described with reference to FIG. 1. A receiving signal received at the antenna 201 is converted into a baseband signal by the radio frequency demodulator circuit 202. When an information of an optimal number of fingers incomes to the receiver from a related transmitting side, the information is extracted by the finger information extractor 204 having a searcher function. The finger information extractor 204 determines one of the finger circuits 203a to 203n which is to be operated, according to the optimal finger number information and sends an operation control signal to the respective finger circuits. When the optimal finger number information indicates a large number, signal components passed through a plurality of paths can be received and reproduced efficiently by a corresponding number of finger circuits. On the contrary, when the optimal number of fingers contained in the optimal finger number information is small and the number of paths is small, unnecessary reproduction and synthesis of noise is prevented and it is possible to maintain S/N ratio high. Although, in the Japanese Patent Application Laid-open No. H8-116303, the optimal finger number information is informed by the other side of the communication after a formation of radio linkage therebetween and before a start of communication, it is also disclosed that the optimal finger number information may be informed periodically or with an arbitrary timing from the other side of communication even after the communication is started.
In the above mentioned prior art, however, reliability of the notice of finger number information from the other side of communication is a problem and an effective RAKE receiving is impossible. It is described in the Japanese Patent Application Laid-open No. H8-116303 that the number of fingers can be changed as occasion may demand. In such case, however, there is a possibility that an information of change of the finger number is received erroneously since the information is noticed after a degradation of receiving condition is started.
Further, the mutual correlation between the receiving signal and the replica of the spreading code may have some value even at a time outside the correct timing due to self correlation characteristics of the spreading code. Therefore, there is a possibility that, when the RAKE receiving is performed by using such mutual correlation value, a timing outside the delay time of a normal path is erroneously deemed as a timing with which the RAKE synthesis is to be performed. Since it is impossible to obtain the expected effect of RAKE synthesis when the synthesis is performed with such erroneous timing, the receiving signal quality is degraded.
On the contrary, when a timing with large self-correlation value is preliminarily deemed as erroneous and a path with that timing is neglected, an effective path having such timing may be disregarded and thus the advantages of the RAKE receiver will be lost.
The present invention was made in view of the above mentioned defects of the prior art and has an object to provide a receiver of a CDMA system utilizing a spread spectrum scheme and, particularly, a receiver which realizes a reliable RAKE synthesis by stably extracting a path timing to be synthesized.